1. Field of the Invention
The present invention relates to various boilers including a steam boiler, a hot water boiler, a heat medium boiler, a waste heat boiler, and an exhaust gas boiler. In particular, the present invention relates to a multitubular boiler including a boiler body and longitudinal fins, the boiler body having a plurality of vertical heat transfer tubes arranged to form a cylindrical shape so as to connect an upper header to a lower header, the longitudinal fins being provided, in at least a part in a peripheral direction of the plurality of the vertical heat transfer tubes arranged to form the cylindrical shape, to gaps between the adjacent vertical heat transfer tubes.
The subject application claims a benefit of the priority of Japanese Patent Application No. 2007-112229 filed on Apr. 20, 2007, and contents thereof are herein incorporated.
2. Description of the Related Art
There are known as multitubular boilers ones disclosed in Japanese Patent Application Laid-open No. Hei 02-075805 (FIGS. 1 to 3) and Japanese Patent Application Laid-open No. 2004-225944. The boiler body of the boiler of this type includes the plurality of the water tubes between the upper header and the lower header each formed in an annular shape. The plurality of the water tubes are arranged in the peripheral direction of the upper header and the lower header so as to form a single row or two rows. In the boiler body including the above-mentioned water tube row, the inside of the inner water tube row is the combustion chamber, and an outside of the inner water tube row is the combustion gas flow path.
When the fuel is burned such that flame is generated from the burner installed in the upper portion of the boiler body toward the inside of the combustion chamber, a combustion gas is reversed in the lower portion of the combustion chamber and passes between the inner water tube row and the outer water tube row or between the outer water tube row and the boiler body cover to be discharged as an exhaust gas to the flue from the upper portion of the boiler body. In the meantime, the combustion gas undergoes heat exchange with water in each of the water tubes. As a result, water in the water tube is heated.
The boiler is normally controlled based on a pressure in the boiler body. Accordingly, temperature of water or steam in the boiler body becomes a saturation temperature at the control pressure, and temperature of the water tubes becomes close to the saturation temperature. On the other hand, the boiler body cover is exposed to the combustion gas or the exhaust gas of high temperature, so the temperature thereof is higher than the water tube temperature. Therefore, in a case where the water tubes and the boiler body cover are made of the same material, due to a temperature difference, there is generated a difference between a thermal expansion amount of the water tubes and a thermal expansion amount of the boiler body cover. As a result, thermal stress acts on the boiler body cover. That is, the thermal stress acts on the boiler body cover due to the temperature difference with respect to the water tubes.